1. Field of the Invention
The present invention relates to a process For the treatment of an aqueous solution mainly containing nitric acid and hydrofluoric acid.
Such solutions can in particular consist of industrial effluents such as metal cleaning baths or can be produced in operations during the nuclear fuel cycle.
The invention proposes a process for the treatment of such solutions making it possible to recover on the one hand the two separate acids, which can thus be recycled, and on the other hand a water purified with respect to said two acids. Such a water can be discharged into the environment if it contains no other contaminating substances.
2. Description of the Invention
The treatment of aqueous solutions containing nitric acid and hydrofluoric acid by distillation is made difficult by the properties of the liquid-vapor equilibrium of the mixture of the two acids.
It is also pointed out that the liquid-vapor equilibrium data on this system only relate to mixtures containing traces of hydrofluoric acid or, at the most, 4M hydrofluoric acid solutions. The literature does not provide information on hydrofluoric acid-concentrated mixtures.
U.S. Pat. No. 3,846,256 describes a process for the purification of nitric acid by the elimination of the fluoride ions contained therein. This process is based on the use of metal ions such as zirconium and aluminium ions. Therefore the volatility of the hydrofluoric acid is lowered by the formation of complexes of the fluoride ion of the metals in question and the separation of a mixture of nitric acid and water by evaporation is then possible. Thus, this process makes it possible to purify the nitric acid, but it suffers from two disadvantages. On the one hand it leads to obtaining a salt-containing effluent and on the other it does not permit the separation and therefore the valorization of hydrofluoric acid.
It has also been proposed to introduce silica for increasing the volatility of the hydrofluoric acid by the formation of fluosilicic acid. According to this method, the hydrofluoric acid is also polluted and cannot therefore be volatilized. The said method is also mainly performed in discontinuous operations, essentially for analytical purposes.